1. Field of the Invention
The present invention relates to a scanning optical system and an image forming apparatus using the scanning optical system, and particularly to scanning optical systems suitably usable in image forming apparatuses, such as laser beam printers, digital copying machines, and multi-function printers that employ electrophotographic process, for example, in which a plurality of light beams emitted by a plurality of light source units are deflected by a polygon mirror serving as a optical deflecting unit, are transmitted through an image forming optical system having f-θ characteristics, and are scanned on a surface to be scanned (a scanned surface) to record image information thereon.
2. Related Background Art
Construction and optical function of a conventional scanning optical system will be described with reference to FIGS. 12 to 14.
FIG. 12 illustrates an image forming apparatus for printing a color image, which includes four independent image bearing members (also referred to as a photosensitive drum) corresponding to colors of Y (yellow), M (magenta), C (cyan) and Bk (black), respectively.
In FIG. 12, reference numeral 20 designates a photosensitive drum. In the photosensitive drum 20, an electrically conductive body is coated with a photosensitive layer, and an electrostatic latent image is formed thereon by a light beam emitted from a scanning optical portion contained in an optical box 21. The scanning optical portion emits plural light beams based on image information supplied from an image reading apparatus (not shown), a personal computer (not shown), or the like. Reference numeral 22 designates a developing unit for forming a toner image on the photosensitive drum 20 with frictional electrified toner. Reference numeral 23 designates an intermediate transferring belt for carrying the toner image on the photosensitive drum 20 to a transferring paper. Reference numeral 24 designates a sheet feeding cassette for containing sheets of paper on which the toner image is transferred. Reference numeral 25 designates a fixing unit for causing adsorption of the toner image transferred on the paper into the paper by heat. Reference numeral 26 designates a sheet discharging tray on which the fixed transferring paper is carried. Reference numeral 27 designates a cleaner for cleaning toner remaining on the photosensitive drum 20.
In connection with image formation, four light beams emitted from the scanning optical portion based on image information are projected on corresponding photosensitive drums, respectively, and electrostatic latent images are thus formed on these photosensitive drums electrified by the electrifying units, respectively. After that, toners frictionally electrified in the developing devices 22 are attached to the electrostatic latent images, and toner images are thus formed on the photosensitive drums 20. The toner images are transferred to the intermediate transferring belt 23 from the photosensitive drums, respectively, and these toner images are again transferred to the paper conveyed from the sheet feeding cassette 24 disposed in a lower portion of the apparatus. The image is thus formed on the paper. The image transferred on the paper is fixed by fixation of the toner using the fixing unit 25, and the paper is stacked on the sheet discharging tray.
FIG. 13 is a sub-scanning cross-sectional view of the scanning optical system illustrated in FIG. 12. In FIG. 13, two scanning groups S1 and S2 are disposed symmetrically with respect to a polygon mirror 28 serving as the deflecting unit in a horizontal direction. Optical functions of those two scanning groups S1 and S2 are the same, and accordingly the following description is made to the scanning group S1 on the right side only.
In the scanning optical system in FIG. 13, plural light beams emitted based on the image information are guided onto the photosensitive drums 20a and 20b through the polygon mirror 28 for deflecting and scanning the light beams, first and second f-θ lenses 29 and 30 for scanning the light beams at uniform velocity and forming spot images on the photosensitive drums, a plurality of reflecting mirrors 31a to 31d for reflecting the light beams toward predetermined directions, and dust-proof glasses 32 for protecting the scanning optical portion from dust, respectively. The latent images are thus formed on the photosensitive drums 20a and 20b, respectively.
In recent years, as the size of the image forming apparatus decreases, the scanning optical system comes to adopt a system in which four photosensitive drums are scanned with and exposed to light beams from a single polygon mirror (a polygon motor unit), respectively, as illustrated in FIG. 13. This system includes two scanning groups S1 and S2 for projecting plural light beams to opposing facets of the polygon mirror, respectively.
Each of the two scanning groups S1 and S2 causes two light beams shifted in the vertical direction a predetermined distance in a parallel manner to be incident on the deflection facet (reflecting facet) of the polygon mirror 28, such that the two light beams can be deflected and scanned. Further, there are provided first and second f-θ lenses 29 and 30 for imaging two light beams E1 and E2 in those upper and lower optical paths on the photosensitive drums 20a and 20b. Each of the first and second f-• lenses 29 and 30 has two identical lens surfaces provided on upper and lower stages. Each of those lenses can be produced by cementing two lenses, or by forming a mold lens in a united form.
The above-discussed conventional apparatus, however, has the following disadvantages.
The first disadvantage is that optical components are independently provided on respective optical paths from plural light sources to the optical deflecting unit for the respective light beams, and accordingly the number of optical components is large. The number of optical components must be decreased to achieve further reduction of costs.
In the two-stage scanning optical system as illustrated in FIG. 13, it is necessary to provide a deflecting facet for deflecting and scanning light beams in the respective optical paths, and hence a thick polygon mirror, or a two-stage polygon mirror is used. In this system, load imposed on a motor for driving the large-sized polygon mirror tends to increase.
In contrast therewith, there has been proposed an oblique incidence scanning optical system using a thinned polygon mirror. In this system illustrated in FIG. 14, laser light beams are caused to be incident on the polygon mirror at different angles in the sub-scanning cross section such that the polygon mirror can be thinned in the vertical direction. Such a scanning optical system in which the laser light beams are caused to be incident on the polygon mirror at different angles in the sub-scanning cross section is generally called an oblique-incidence system. In FIG. 14, each laser light deflected and scanned by the polygon mirror is transmitted through common f-• lenses 35 and 36, and projected on the photosensitive drum through two folding mirrors and a concave mirror 34b or 34e. 
Scanning optical systems using such an oblique-incidence system are disclosed in Japanese Patent Application Laid-Open Nos. H02-58014, H09-258126, and H11-119131, for example.
In the system of Japanese reference H02-58014, cylindrical lenses on a front side of the optical deflecting unit are separately provided, and consideration for further reduction in the number of optical elements and reduction in cost is insufficient. On the other hand, Japanese reference H09-258126 discloses a system in a cylindrical lens is commonly used, but influence of spherical aberration of the cylindrical lens threatens to occur since the light beam passes through a decentering portion of the cylindrical lens from the optical axis.
The second disadvantage is a method in the oblique-incidence system for separating light beams deflected by the optical deflecting unit from each other in a space between the optical deflecting unit and the scanned surface. Japanese reference H11-119131 discloses a method for coping with such a problem, in which the light beams incident on the deflecting facet from an upper side and a lower side are separated on the deflecting facet, and optical-path separation on a rear side of the optical deflecting unit is hence facilitated. In this system, however, the optical system on the light source side of the optical deflecting unit is liable to be arranged in a cramped fashion. Accordingly, a sufficient space must be secured by making the light source unit remote from the optical deflecting unit, and it is hence difficult to achieve a compact structure.